Digital Frequency Division Bat Detector: Input Amplifier and Comparator
We had chosen to use a CMOS hex inverter IC to build our amplifier, and early investigation of a single stage amplifier showed it could work.
We need much more gain than a single stage could provide, so we cascaded three stages as shown here.
Three stage preamplifier
By experiment we find Acl = 920 at input frequency Fin of 1kHz and Acl = 640 at Fin = 100kHz.
The three stage amplifier is suitable both in terms of available gain and bandwidth (140kHz) as an input preamp for our detector. Apart from the IC it uses only four components. (Not shown: supply decoupling C 0.01uF)
Note that as the input is AC coupled via C1 the circuit is self-biasing into its optimum DC condition. An alternative design used three separate stages each designed for a gain of ten and then cascaded. The circuit used lots more components and no benefit was noted, so we use the simpler circuit.
Aol = 22 for a single stage, so ignoring loading effects Aol = 22 * 22 * 22 = 10,000 approx.
Acl = (Rf/Ri+Rf) * (Aol/(1+BAol) where B = Ri/(Rf+Ri) = 0.01/10.01 = 0.001 to three significant figures
whence Acl = (10/10.01)*(10,000/(1+10,000*0.001)) = 0.9990 * 909 = 908
Schmitt trigger comparator circuit
Noise on our signal could cause mis-triggering, so we use a comparator circuit to produce a square wave signal to go to our counter.
Voltage response of Schmitt trigger circuit
Our Schmitt trigger circuit provided this response. The output would go high (green line) as the input voltage passed through 4.29 volts. It would remain high until the input voltage fell below 4.27 volts. This means we need a minimum signal of about 5 * (4.29 - 4.27) = 0.1V peak to peak from our preamplifier.
Below you can see an illustration of why we need hysteresis to clean up the signal before passing it to the counter.
A noisy signal (just what we expect from a sensor) will produce false counts
Adding a Schmitt trigger circuit to our detector will eliminate this mis-triggering, and give a much more reliable output.